Question 27

College answer

Syllabus topic/section:

2.1.3    Sepsis and Infections - Sepsis and Septic shock: L1
2.1.21 Applied Pharmacology in Intensive Care – Intravenous fluids
2.1.4    Cardiovascular Intensive Care - Haemodynamic monitoring: L1

Discussion: 

Candidates are again reminded to be familiar with and follow the glossary terms, in this instance ‘discuss’ and ‘outline’. Discuss requires detailing of the advantages and disadvantages of the nominated dynamic measures. Outline requires more than a simple list
Less successful answers in part a) listed non-dynamic measures for example CVP, or simply gave a fluid bolus, which did not attract marks.
Successful answers nominated Passive leg raise, various echo measurements, haemodynamic measurements affected by respiration and ventilator dynamic measures such as tidal volume challenges amongst others.

Candidates who scored more marks in part b) demonstrated a structured approach and were then less likely to omit details that attracted marks. The more successful candidates also timed their answers between parts a) and b) appropriately for the marks stipulated.

Discussion

"Discuss" implies advantages disadvantages and controversies. This is hard to achieve concisely, for six marks. A tabulated response may not necessarily be the best.

PPV and SVV

• based on changes in cardiac output associated with mechanical ventilation (for PPV, 12% threshold seems to be well supported)
• Invalidated by arrhythmias, the presence of right heart failure, cardiac tamponade, etc
• Ideally requires patients on mandatory ventilation with large tidal volumes (10ml/kg)
• "grey zone" values between 9% and 13% have very poor sensitivity/specificity

Passive leg raise autotransfusion

• Represents a reversible fluid challenge
• Well validated
• The patient needs to have both legs intact, an intact pelvis, no raised ICP, and needs to be ventialated with a mandatory mode

IVC assessment by bedside echo    

• M-mode from the subcostal view can assess the IVC distensibility index, with 18% as the best cut-off (% variation in diameter)
• This is a reflection of the changes in intrathoracic pressure imposed by breathing
• Poor predictive performance in spontaneously breathing patients
• Unreliable where there is increased abdominal pressure
• Impossible without good subcostal views

Respiratory variations in aortic blood flow velocity

• TOE Doppler of the descending aorta
• Delta VPeak, the percentage change in peak aortic blood flow velocity
• during the respiratory cycle, is the main measure
• 12% is again the value with peak specificity and sensitivity
• Requires TOE (not exactly minimally invasive)
• Affected by aortic compliance
• Affected by the same factors as SVV and PPV

LVOT VTI    

• Approximation of stroke volume by TTE, where measurement of LVOT diameter and the velocity of aortic flow is used to calculate the stroke volume
• Similar utility to PPV and SVV, but non-invasive
• Ultrasonography skills are required
• Minor interobserver variations in technique are magnified by the calculations
• Serial reassessment may be time consuming

b) Potential complications of fluid over-resuscitation

• Complications related to volume:
  • Organ oedema:
    • Impaired respiratory function and delayed ventilator weaning
    • Impaired wound healing
    • Fluid collections, eg. pleural effusions, that act as reservoirs to trap haematogenously spread infection
    • Decreased renal and hepatic perfusion
    • Increased gut permeability
    • Associated with increased mortality (FEAST, SAFE, SOAP, VASST )
• Complications related to fluid composition:
  • Hyperchloremia (saline)
  • Hyponatremia (others)
  • Alkalaemia (balanced crystalloids)
  • Dilution of haemoglobin and clotting factors